1. Field of the Invention
Aspects of the present invention relate to a device for mixing and storing fuel and water in a fuel cell system, and more particularly, to a device for mixing and storing fuel and water in a direct methanol fuel cell (DMFC) system for a portable application.
2. Description of the Related Art
A fuel cell system generally comprises a fuel cell stack, a heat exchanger, a water separator, a mixer, a carbon dioxide separator, and a tank.
In the fuel cell system, concentrated fuel and pure water are mixed into a fuel mixture. In a mixer, concentrated fuel from a fuel tank, water recovered from a heat exchanger, unreacted fuel and water from an anode are mixed and stored. Through an outlet of the mixer, the mixture of fuel and water flows to the fuel cell stack.
A standard direct methanol fuel cell (DMFC), similar to those disclosed in Matsuoka et al. (US 2004/0166389 A1) and (US 2004/0062964 A1), is shown in FIG. 1. A DMFC stack 10 comprises a plurality of fuel cells, each fuel cell having a cathode 19 with a cathode air inlet 11 and a cathode air outlet 13; an anode 17 with an anode inlet 15 and an anode outlet 16; and a membrane 14 separating the anode 17 and the cathode 19. An air pump 12 supplies reaction air to the cathode 19 through the cathode air inlet 11. The DMFC stack 10 produces an electrochemical reaction a between a fuel stream composed of water and methanol to the anode 17 and an oxidant stream (air or oxygen) supplied to the cathode 19. The DMFC stack 10 directly produces electricity through an electrochemical reaction. The membrane 14 of the DMFC stack 10 is permeable to protons and thus the operation of the DMFC stack 10 generates a transfer of protons from the anode 17 to the cathode 19. Methanol and water enter the anode 17 of the fuel cell stack 10, and hydrogen is catalytically split from the methanol to produce carbon dioxide and free electrons. The electrons, as the membrane 14 is only permeable to protons, are conducted to the cathode 19 through an external circuit. Such flow of electrons through an external circuit creates a useable current. As the current produced by each individual fuel cell is not great enough for the needs of many applications, the individual fuel cells are arranged to form the fuel cell stack 10. At the cathode 19, the supplied oxygen, generally from air, is combined with protons that flow through the membrane 14 and the free electrons that flows through the external circuit to form water. Furthermore, the membrane 14 remains hydrated as a result of electroosmotic drag wherein the migrating protons effectively pull water molecules into the membrane 14 as the protons are transferred through the membrane 14 from the anode 17 to the cathode 19.
In order to use a concentrated fuel and minimize the volume of a fuel tank 30, the recovery of the water produced at the cathode 19 is essential to a DMFC system. A heat exchanger 50 is used to condensate the water from the cathode air outlet 13. The water is separated in a water separator 60 and is recycled through an anode circuit 18. The air exits the water separator 60 at a venting opening 61. A liquid outlet 62 of the water separator 60 is connected to the main anode circuit 18. The anode fuel circuit 18 is composed of a circulation pump 23 to feed the fuel mixture to an anode 17 through the anode inlet 15, the anode outlet 16 is connected to a carbon dioxide separator 20, and a mixer 40. The anode outlet 16 is connected to the carbon dioxide separator 20 that separates carbon dioxide generated by the fuel cell reaction from the unreacted methanol and water. The carbon dioxide may be vented to the atmosphere or captured for other uses through a carbon dioxide outlet 21. The unreacted methanol and water are recycled back to the mixer 40 through the recycle outlet 22. The mixer 40 mixes a concentrated methanol fuel from the fuel tank 30, the water recovered from the heat exchanger 50 by the water separator 60, and the recycled methanol and water from the carbon dioxide separator 20. A driving force is necessary to feed the water recovered from the heat exchanger 50 and the water separator 60, therefore the DMFC system has a pump 63 supply the water recovered from the heat exchanger 50 and the water separator 60 to the mixer 40. And, the concentrated fuel is delivered from the fuel tank 30 to the mixer 40 by a pump 31.
A device combining the function of a carbon dioxide separator, a water separator, a mixer, and a tank is described in Muller et al. (EP 1383190 A1) and (EP 1383191 A1). The principle of the device is shown in FIG. 2. A water/air inlet 101 is located on the top part of the device. The liquid water is separated by gravity 106 and falls to the bottom of the device. The fuel inlet 102 that supplies fuel to the device is located at the bottom of the device that works as a liquid hold-up tank. The gases from the incoming stream through the fuel inlet 102 exit the device through an outlet vent 104 which is a liquid-tight gas permeable membrane. A liquid mixture exits the device through an outlet 103 and flows to the fuel cell stack. The above-described device fails as both a mixer and a tank as the operation of the device is dependent on the physical orientation of the device so that the use of the device in a portable system is restricted to one position or orientation. That is, the device as disclosed by Muller et al. works only in an up-right position because it utilizes gravity to separate the liquid mixture from the gas. Thus, use of the device in a tilted or in an inverted position is either impossible or such use would require additional safety installations to protect the device against malfunction.
Additionally, the functional principle (combination of the carbon dioxide separator, the water separator, the mixer, and the tank) of the device leads to a significant volume and a large height that makes it difficult to integrate a DMFC system in a laptop docking station, battery-like system, or other small system.